Add testcase for EWMH desktop viewport

Patch status: merged

Patch by Tony Crisci

Long description:

Test that the EWMH specified property _NET_DESKTOP_VIEWPORT is updated
properly on the root window. We interpret this as a list of x/y
coordinate pairs for the upper left corner of the respective outputs of
the workspaces.
This test is for this commit:
feature: implement ewmh desktop viewport property
4205973135f28a0ab0e25a63dbc8c6fff0d3539e